The overall aim of this project is to develop PET cameras whose geometry is optimized for detecting breast cancer or axillary node involvement. These post-mammography tools would determine whether suspicious structures observed in mammograms have the increased metabolism or chemistry associated with breast cancers, as well as image the metabolic activity in the axilla to determine the extent of axillary node involvement. The instrumentation proposed has the potential to provide a cost effective, non-invasive alternative to biopsy as well as accurate information on axillary node involvement that is not available from existing techniques. The proposed technique relies heavily on the fact the FDG is an excellent tracer for breast cancer, with less than ninety percent specificity and selectivity and a tumor to normal tissue contrast ratio of 8:1, as measured with conventional PET imaging. The proposed instruments have higher sensitivity and spatial resolution than conventional PET cameras, and so allow rapid identification of cancerous lesions and axillary involvement for structures down to 5 mm in size with a small (less than 1 mCi) injected dose of FDG. The project focuses on development of the necessary detector technology and reconstruction algorithms culminating with images of phantoms - no patients or animals will be imaged. The cameras are based on a detector module design consisting of a large number of small LSO scintillator crystals, each coupled on one end to a photomultiplier tube (which provides a timing pulse and energy discrimination) and on the other end to an individual silicon photodiode (which identifies the crystal of interaction and measures the depth of interaction within the crystal). Thirty-six such detector modules are placed in close proximity to the breast (similar to a mammography unit) or the axilla (similar to a small diameter PET ring). These geometries improve the sensitivity and the spatial resolution significantly compared to a conventional PET camera - the sensitivity increase is a factor of 4-30 for the breast and a factor of 1-10 when imaging the axilla. The small number of detectors reduces the camera cost by an order of magnitude compared to a conventional BET camera, and when combined with the lower amount of radio-pharmaceutical needed, this development could reduce the cost of a patient examination significantly. These cameras are designed specifically for breast cancer, but the detector module developed could also be incorporated into high resolution PET cameras with conventional geometries. These designs, when complete, will be offered to private industry to incorporate into a clinical instrument.